This invention relates to a circuit arrangement for energizing a discharge lamp comprising;
input terminals which are to be connected to a DC voltage source,
an inverter coupled to the input terminals for generating a lamp current from the DC voltage supplied by the DC voltage source, which inverter comprises
a switching element coupled to the input terminals,
a control circuit coupled to a control electrode of the switching element, which control circuit serves to generate a control signal for rendering the switching element alternately conducting and non-conducting, and
a pulse duration modulator which is coupled to the control circuit and which sets the duty cycle of the control signal duty cycle being directly proportional to a digital signal present at an output of the pulse duration modulator.
Such a circuit arrangement is well known. In such a circuit arrangement, the duty cycle of the control signal can be set in a readily reproducible manner, independent of, for example, the ambient temperature. A drawback of such a circuit arrangement is, however, that not every value of the duty cycle of the control signal can be set since a digital signal is composed of a limited number of bits. As a result, also the power consumed by a lamp energized by means of the circuit arrangement has only a comparatively small number of settings.
It is an object of the invention to provide a circuit arrangement enabling not only the duty cycle of the control signal and hence the power consumed by a discharge lamp energized by means of the circuit arrangement to be very reproducibly adjustable, but also enabling the average value of the duty cycle of the control signal and the average value of the power consumed by the lamp to be set to a comparatively large number of settings.
To achieve this, a circuit arrangement as mentioned in the opening paragraph is characterized, in accordance with the invention, in that said pulse duration modulator is further provided with a circuit part M for periodically modulating the digital signal, each period of this modulation comprising a first time interval wherein the digital signal has a first value, and a second time interval wherein the digital signal has a second value, said first and said second value being independently adjustable by the circuit part M.
The modulation of the digital signal leads to a modulation of the duty cycle of the control signal and to a modulation of the power consumed by the lamp. If the first and the second value of the first digital signal are chosen to be different, the value of the duty cycle of the control signal and hence the power consumed by the lamp during the first time interval corresponds to the first value of the digital signal and, during the second time interval, to the second value of the digital signal. The average power consumed by the lamp in a period of the modulation ranges between the value of the lamp power corresponding to the first value of the digital signal and the value of the lamp power corresponding to the second value of the digital signal. By virtue thereof, the average value of the lamp power can be set to a number of settings exceeding the possible number of values of the digital signal.
Preferably, each period of the modulation comprises N successive time intervals, N being a natural number larger than or equal to 2, and the value of the digital signal during at least one of these time intervals can be set by the circuit part M at a value that differs from the value during one of the other time intervals. The number of possible settings of the lamp power increases as the value chosen for N increases.
The circuit part M for periodically modulating the digital signal can be embodied so as to be comparatively simple if each one of the N time intervals is of equal duration. Preferably, the circuit part M comprises a timer for xe2x80x9ctimingxe2x80x9d the successive time intervals.
The circuit part M may be additionally provided, however, with a circuit part Mxe2x80x2 for setting the duration of one time interval or of each one of the successive time intervals. By setting the duration of at least one of said successive time intervals, it is possible to set the average value of the duty cycle of the switching elements and hence the average value of the power consumed by the lamp. In this case, N is preferably equal to 2 because this enables the structure of the circuit part Mxe2x80x2 to be comparatively simple. Setting the duration of one time interval or of each of the time intervals in a modulation period is particularly advantageous in embodiments of a circuit arrangement in accordance with the invention wherein a microprocessor is used to form the circuit part M. It has been found that a high resolution of the adjusted lamp power can be brought about by using only a small part of the xe2x80x9cCPU timexe2x80x9d of the microprocessor.
In a preferred embodiment of a circuit arrangement in accordance with the invention, the inverter does not comprise a single switching element but a bridge circuit provided with a series arrangement of a first switching element and a second switching element, which series arrangement also interconnects the input terminals, and outputs of the control circuit are coupled to respective control electrodes of the switching elements, and the control circuit generates a first control signal and a second control signal for rendering, respectively, the first and the second switching element conducting and non-conducting. This preferred embodiment can be embodied such that the duty cycles of the first and the second control signal are equal and directly proportional to the digital signal present at the output of the pulse duration modulator. It is alternatively possible, however, to modulate the first and the second control signal in the same manner and, subsequently, subject the first control signal to a phase shift relative to the second control signal. This phase shift does not influence the lamp power, but causes the modulation of the luminous flux of the lamp resulting from the modulation of the duty cycle of the control signal to be suppressed.
The preferred embodiment can also be embodied such that the duty cycles of the first and the second control signal can be independently modulated. Instead of one pulse duration modulator, the circuit arrangement is provided, in such an embodiment, with a first pulse duration modulator for setting the duty cycle of the first control signal and with a second pulse duration modulator for setting the duty cycle of the second control signal, the duty cycle of the first control signal being directly proportional to the value of a first digital signal present at an output of the first pulse duration modulator, and the duty cycle of the second control signal being directly proportional to the value of a second digital signal present at an output of the second pulse duration modulator, the first pulse duration modulator being provided with a first circuit part M1 for periodically modulating the first digital signal, and the second pulse duration modulator being provided with a second circuit part M2 for periodically modulating the second digital signal. In such an embodiment of the preferred embodiment, the average value of the duty cycle of the first control signal can be chosen to be different from the average value of the duty cycle of the second control signal, as a result of which the number of settings to which the lamp power can be set is increased further. The modulation frequencies of the first and the second control signal can be chosen to be equal or unequal.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.